This invention relates to the conversion of energy from naturally occurring sources of mechanical energy, and particularly to the conversion of the mechanical energy present in ocean surface waves to useful energy, particularly electrical energy.
In many known systems for capturing surface wave energy, a float is used for being vertically oscillated in response to passing waves. The float is rigidly coupled to an energy converter which is driven in response to vertical movements of the float. In one system, described in U.S. Pat. Nos. 4,4,773,221 and 4,277,690 (the subject matter of which is in herein by reference), an open-ended hollow tube is rigidly suspended beneath a float, the tube being completely submerged and in vertical orientation.
The tube vertically oscillates in the water in correspondence with movements of the float and, in the absence of anything within the tube, the tube moves freely relative to the column of water within the open-ended tube. In one embodiment, a movable piston is disposed within the tube for blocking relative movements between the water column and the tube. As the tube and float oscillate within the water, the mass of water within the tube tends to block corresponding movements of the piston, hence the piston moves relative to the tube. Actual movements of the piston do occur, however, and provided the entire system is oscillating at its natural resonant frequency, relatively large amplitude oscillations of the piston can occur. The moving piston drives an energy converter fixedly mounted, e.g., within the float, for converting the piston movements to useful energy.
While these float driven tube systems work, efficient operation requires that the natural resonant frequency of the system closely matches the frequency of the ocean waves driving the system. While this can be generally accomplished at a specific site and specific time, particularly if means for adjusting the resonant frequency of the system in response to changing surface wave frequencies are provided, a problem is that, at any instant, multiple random surface waves are present whereby much of the wave energy present can not be efficiently transferred to the oscillating system. Also, the means for adjusting the resonant frequency of the device generally involves changing the water mass within the device. Since this mass is quite large, it is not readily changed.
A feature of the present invention is that a relatively high efficiency of operation is obtained which is relatively insensitive to random variations of wave frequencies and amplitudes.
In a first embodiment of the invention, an open-ended, hollow tube is disposed in vertical, submerged and fixed location relative to the mean water level. Specifically, the tube is not inxe2x80x9cfloatingxe2x80x9d (moveable) relationship with the passing waves. The length of the tube and the depth of the top end of the tube beneath the mean water level are selected, as described hereinafter, depending upon the frequency and amplitude of the most prevalent anticipated surface waves, as well as the water depth. While maximum efficiency of operation is attained when the anticipated waves are present, the fall-off of efficiency of operation is relatively small with variations of wave conditions.
During operation, pressure variations, at the top, open end of the tube (caused by passing waves) in comparison with a relatively fixed pressure at the open, bottom end of the tube (unaffected by passing waves) cause vertical flows of water through the tube which are used for driving an energy converter, preferably by means of a movable piston within the tube.
In a second embodiment of the invention, a hollow tube having a closed top end and a bottom open end is disposed in vertical, submerged but relatively movable relation with the mean water level. In a preferred embodiment, the tube is secured for vertical cyclical movements relative to a float fixedly submerged beneath the water surface and disposed within the tube. The dimensions of the tube and its at-rest location relative to the water surface are in accordance with the tube of the first embodiment except, in the second embodiment, the piston movable within the tube of the first embodiment comprises the closed top end of the tube of the second embodiment. During operation, pressure variations against the top end of the tube cause vertical oscillations of the tube relative to the fixed float, and such oscillations are used for driving an energy converter fixed within the float and coupled to the movable tube. In a variation of the second embodiment, the transducer is not mounted in a float but fixedly secured to the ocean bottom. The movable tube, which need not be hollow, is secured to the transducer
In all embodiments, movements of the tube relative to the adjoining water are caused not by wave-induced displacements of a float on the water surface, but in response to pressure variations caused by passing waves.